Lamp burnout detector



July 21, 1959 A. E. GLANDQN 2,896,121

LAMP BURNOUT DETECTOR Filed Dec. 11, 1957 Adrian/E Glandon IN VEN R.

WM WMM United States Patent LAMP BURNOUT DETECTOR Adrian E. Glandon, Cedar Rapids, Iowa, assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Application December 11, 1957, Serial No. 702,055

4 Claims. (Cl. 315-119) The present invention relates to electric lamps that are operated by direct current, and more particularly concerns the automatic extinction of such lamps in response to the occurrence of arcs therein.

In photographic processes, it is frequently necessary to use a light source of precisely controllable intensity. One light source particularly useful for its ease and speed of intensity control is a filamentary lamp operated as the load resistance of a vacuum-tube amplifier. Direct current flows through the lamp because of the unidirectional flow of plate current in the vacuum tube, and because of the direct-voltage supply. The primary advantage of this circuit is that the lamp current, which is the plate current for the vacuum-tube amplifier, may be controlled by a low-power signal source connected to the control grid of the amplifier tube. Furthermore, with a well-filtered supply voltage, the lamp current, and consequently the lamp intensity, may be made relatively free from rapid variations such as occur when a lamp is operated by 60-cycle alternating current.

The unidirectional flow of lamp current engenders a problem not encountered when a-filamentary lamp is operated by alternating current. While a lamp of this type is operating, its filament material slowly boils or evaporates away and the diameter of the filament gradu ally decreases. Ultimate failure of the lamp normally occurs when the weakened filament separates and begins to are at some point in its length, either through a continuation of the material loss or through mechanical failure. If the filament is heated by alternating cur rent, the arc is extinguished when the current ceases to flow at the end of a half cycle. On the other hand, in a lamp operated by direct current, the arc persists until it has burned away suflicient material so that the gap is too long to sustain the arc with the available power. For a typical lamp design, the arc in a D.C. system may persist until the lamp, the socket and portions of the connecting wiring and supporting structure are consumed, with consequent damage to the instrument in which the lamp is installed.

It is therefore a principal object of the present invention to detect the occurrence of an arc in a lamp operated by direct current and to extinguish the are before damage has occurred.

Another object of the invention is to detect a shift in the distribution of power dissipation as between an amplifier and a filamentary lamp that constitutes a resistance load of the amplifier.

A further object is to detect an increase of predetermined magnitude in the power dissipation of an amplifier which drives a filamentary lamp.

Another object is to add analogs of the anode voltage and anode current of an amplifier to produce an analog sum which is an approximation of the power dissipation of the amplifier, and to compare that sum with a predetermined value for initiating a control function when a predetermined result of the comparison occurs.

Other objects of the invention will appear from the the plate current-plate voltage relationship as the plate current is varied in response to variations in grid voltage, and with a resistance load R of constant value, so that The curve E '-l represents the corresponding relationship with a filamentary lamp as load, where the lamp resistance change is considerable as the lamp current is varied, causing a non-linear relationship between V and i Normal steady-state operation of the lamp control circuit would follow the line E i,, However, when a rapid increase in lamp intensity is desired, as in operation of a high-speed photographic printer, a transient condition occurs wherein a large positive signal is applied to the control grid of the tube. In such case the internal resistance of the tube becomes very low, and

' across the lamp filament resistance.

the lamp resistance is largely effective in determining the current flow in the circuit. Line E l represents this transient condition, where the line slope corresponds to the cold resistance of the lamp, and i represents the current flow with voltage E applied almost entirely Normal operation, then, is defined by the two lines E -i,, and Eg-im,

When an arc is formed across a gap in the filament of the lamp, an increasing portion of the lamp filament is replaced by the are as the filament material evaporates or melts away to sustain the arc. Therefore, a portion of the normal filament resistance is replaced by the arc resistance. The volt-ampere relationship of an arc is a complex function of current density, materials, gaseous pressure and length of the are path. However, at con-' stant current and for a given path length, the voltage drop is substantially constant.

At the instant an arc occurs, the voltage drop across the equivalent length of filament is less than the voltage drop of the arc. Therefore, the total voltage drop across a broken filament, including the arc, is initially greater point is reached at which the arc voltage drop is less than the voltage drop of the filament length destroyed; beyond that point, the effective resistance of the lamp decreases. Assuming that the lamp operates at constant current during formation of the arc-an assumption discussed hereinafter-At will be evident that the power dissipation of the lamp decreases as its resistance decreases. Since the lamp is in the plate circuit of an amplifier tube, the plate current of the tube also is constant for a given value of signal applied to the control grid of the amplifier. Therefore, if the supply voltage for the over-all lamp-amplifier circuit is constant, a constant total power is dissipated in that circuit:

where P =total power dissipated in tube and lamp;

P =power dissipated in tube; P =power dissipated in lamp; and V =voltage drop across lamp When the lamp resistance decreases due to an arc,

the portion of the total power dissipated in the lamp decreases, because it is proportional to lamp resistance. P =V i ,=-i R lamp (3) Therefore, the power dissipated in the tube increases. P =P -P V i (4) The above analysis is based on' the assumption of 'constant current in the amplifier tube and lamp. This assumption is substantially justified where the amplifier tube has a screen grid, as shown in the amplifier 10 in Fig. 2. In such tubes, the flow of plate current is controlled primarily by voltages applied to the control grid 12 and the screen grid 14, and is relatively independent of plate voltage.

It can be shownthat in a circuit employing a triode amplifier tube, the action of the arc in changing the distribution of voltage and power in the circuit is in the same direction, and it may be more pronounced than in a circuit employing a tube having a screen grid. In a triode,'the plate current is dependent on both grid voltage and plate voltage. When the plate voltage increases due to decreased lamp voltage, the plate current, i.e., the current through the lamp, increases. It is known that one characteristic of an arc is that its incremental resistance is negative, that is, an increase of current in the arc is accompanied by a decrease in the voltage drop in the arc. Therefore, if the arc has destroyed an appreciable portion of the lamp filament, the increase in current through the tube and lamp causes a further decrease in lamp voltage, and the plate voltage of the tube is further increased.

In Fig. l, the curve labeled P max represents the boundary of the region within which allowable tube power dissipation, as established from the manufacturers ratings, occurs. It would be possible to measure the actual power dissipation of the tube as an indication of the distribution of the total power between lamp and tube. However, the normal operating point of the tube may be located at any point along the curve E i,, of Fig. 1, depending on grid voltage. This produces a varying margin between normal operation and the boundary operation as defined by the curve P In addition, calculation of the tubes power dissipation requires multiplication of the values of plate voltage and plate current, a somewhat difficult computation to perform with simple electrical circuitry. Since the area of normal operation is bounded by the straight line E -4 12, 8 parallel straight line, such as that labeled T =(aV,,}-bi,,) in Fig. 1, is entirely valid as an arbitrarily selected boundary beyond which abnormal or unsafe operation of the tube can be assumed, allowing for circuit tolerances. In addition, the calculation of a straight-line equation need involve only summation of the appropriate electrical quantities, in this case the quantities V and i,,. The burnout detection then consists of determining whether the sum of these electrical quantities is more than the allowable value.

Referring to Fig. 2, the lamp 15, having a filament 16, is connected in series with the anode of amplifier tube 10, and is connected to the positive terminal of a floating voltage source E through a normally closed switch 18. Although amplifier 10 is shown and described as a single tube, it is often preferred, in practice, to use a number of such tubes connected in mutual parallel relation. A pair of resistors R and R are connected in series between ground and the junction of lamp 15 and tube or tubes 10. The junction of resistors R and R is connected to one control-grid input of a differential amplifier 20 comprising a twin triode. Amplifier 20 is of the class of devices designated broadly as summing amplifiers. It will be seen that the voltage V at the junction of resistors R and R is an analog of the plate voltage V of tube 10:

V -aV,,- V,,

where a is a constant. The cathode of tube isv connected to ground and is connected through a resistor R to the negative terminal of the floating anode supply E The terminal E also is connected to the second grid input of the difierential amplifier 20. It will be seen that with respect to the cathode of tube 10, the voltage V at terminal E;; is a negative analog of the plate current i of tube 10:

where b is a constant.

By proper choice of the parameters R R R and E the analogs V and V may be made comparable in magnitude.

In the circuit shown in Fig. 2, screen-grid current flows through resistor R but this current may be neglected in comparison with the anode current i In a circuit employing a triode tube, only i flows through resistor R In the differential amplifier 20, the plate of the lefthand tube, which tube receives input V is connected directly to the positive terminal of a supply B while the plate of the righthand tube, which receives input V is connected to +E through a resistor R The cathodes of both triodes in amplifier 20 are joined and are connected through a resistor R to the negative terminal of supply E It will be apparent that e the voltage at the plate of the righthand triode in amplifier 20, is the sum of the absolute values V and V or the difference between the algebraic values thereof:

Since V is a negative analog of i e is an analog. of the sum of the absolute values of aV and bi The output 2,, from amplifier 20 is compared with the maximum allowable value T=(aV,,-lbi which defines the limit line shown in Fig. l, and if 2 exceeds T, a lamp burnout is detected. This detection is employed to actuate a circuit, next described, to remove power from the lamp circuit or otherwise stop the flow of current through the lamp. The are is thereby extinguished, plate current ceases to flow in tube 10, the signal voltages V and V fall to zero, and both the lamp-control circuit and the burnout-detection circuit become inactive.

The output of the differential amplifier 20 is fed through a resistor R to the control grid of a thyratron tube 22 that is biased below conduction by a voltage increment which is numerically equal to T, the maximum allowable output from the differential amplifier. The bias circuit includes a resistor R, that connects the control grid of thyratron 22 to a tap point on a potential divider R The potential divider R is connected between the cathode of the thyratron and the negative terminal of a supply E for the thyratron. The plate of thyratron 22 is connected through a relay 24 to the positive terminal of supply E Relay 24, when actuated, opens switch 18 in the circuit of tube 10 and lamp 15. Thyratron 22 is prevented by its bias circuit from conducting until the differential-amplifier output e exceeds the allowable value T, at which time the sum of c and the bias voltage through resistors R and R cause the thyratron to conduct. When the thyratron conducts, relay 24 is actuated to open switch 18 and remove power from the lamp circuit. It will be understood that switch 18 need not be located in the plate circuit of tube 10. Instead, it may be interposed between terminal 14 and the screen grid for removing screen bias when relay 24 is actuated. In the latter case there is no excessive screen current as may occur with the arrangement shown in Fig. 2. In the appended claims, where a switch is defined as being in series with the anode circuit of the amplifier tube, it is intended that location of the switch in a screen circuit of the tube is fully equivalent for the purposes of the invention.

Although the invention has been described in its specific application for protecting a power amplifier when the latter has a load comprising a filamentary lamp, it will be understood that the invention has equal application to the protection of any amplifier or regulated power supply from excessive plate dissipation, regardless of the type of load or whether the load is connected on the plate side or the cathode side of the amplifier.

The following parameter values are typical only and are not a unique set for operability of the invention.

Supplies:

E 500 volts D.C. B 300 volts D.C. E 150 volts D.C. Grid 14 250 volts D.C. Tubes:

10 40 type 807 tubes in parallel. 20 12AX7. 22 2D21. Resistors:

R 0.5)(10 ohms. R2 10 Ohms- R 0.125 ohm. R 0.22 l ohms. R 0.22 ohms. R 0.82 1O ohms. R 0.27 10 ohms. R 10 ohms. Lamp 1000 watts, 250 volts.

I claim:

1. In combination with a circuit comprising a circuit load in series with (a) a source of direct current, (b) an amplifier for operating said load, and (c) a normally closed switch, means for detecting a decrease in the impedance of said load for opening said switch, said detecting means comprising: means connected to said circuit for developing a first signal which is an analog of the current flowing through said load and said amplifier; means connected to said circuit for developing a second signal which is an analog of the voltage across said amplifier; means connected to both of said developing means for summing said first and second signals; means connected to said summing means for comparing the sum of the values of said first and second signals to a signal having a predetermined value; and means interconnecting said comparing means and said switch for opening the latter in response to a predetermined result of said comparison.

2. Apparatus for detecting the occurrence of an arc in a filamentary lamp operated by a source of direct current and for disconnecting the lamp from the current source in response to the detection of an arc, said apparatus comprising: a series-connected circuit including (a) said lamp, (b) said current source, (0) a normally closed switch, (d) an amplifier having an anode and a cathode, and (e) a resistor for said amplifier, said resistor being connected between the current source and said cathode, said lamp being connected between the current source and said anode; a source of reference potential; a potential divider interconnecting said last named source and said anode; a summing amplifier having two input terminals and an output terminal; means connecting an intermediate point on said potential divider to one input terminal of said summing amplifier; means connecting the current-source side of said resistor to the other input ter minal of said summing amplifier; a comparison circuit connected to the output terminal of said summing amplifier and adapted, upon receipt of an output signal of predetermined amplitude from said summing amplifier, for generating a control signal; and a normally de-energized relay connected to said comparison circuit and adapted to be energized in response to the generation of said control signal, said relay having an operating connection with said switch for opening the switch in response to energization of the relay.

3. The apparatus defined in claim 2, wherein said summing amplifier comprises a pair of triodes, each triode having a respective control grid constituting a respective one of said input terminals to the summing amplifier; a respective anode in each triode; a second source of direct current connected directly to the anode of the first triode; a resistor connecting said second source of direct current to the anode of the second triode; a respective cathode in each triode; a resistor connecting the cathodes of both triodes to said second source of direct current; and a terminal connected to the anode of the second triode and constituting the said output terminal of the summing amplifier.

4. The apparatus defined in claim 2, wherein said comparison circuit comprises a normally nonconducting thyratron with means for biasing said thyratron below conduction by a voltage of said predetermined amplitude; and an anode circuit for said thyratron, said anode circuit including said relay.

References Cited in the file of this patent UNITED STATES PATENTS 2,614,227 Bordewieck et al Oct. 14, 1952 

